and marshes then outcrop along the shore, (e.g. Kraft, 1971.) Many barrier 

 islands along the Atlantic and Gulf coasts contain tidal and marsh deposits 

 at or near the surface of the littoral zone. The fine material is often 

 bound together by the roots of marsh plants to form a cohesive deposit 

 that may function for a time as beach protection. 



4.24 CONSOLIDATED MATERIAL 



Along some coasts, the principal littoral materials are consolidated 

 materials, such as rock, beach rock, and coral, rather than unconsolidated 

 sand. Such consolidated materials protect a coast and resist shoreline 

 changes. 



4.241 Rock. Exposed rock along a shore indicates that the rate at which 

 sand is supplied to the coast is less than the potential rate of sand 

 transport by waves and currents. Reaction of a rocky shore to wave attack 

 is determined by the structure, degree of lithification, and ground water 

 characteristics of the exposed rock, and by the severity of the wave 

 climate. Protection of eroding cliffs is a complex problem involving 

 geology, rock mechanics, and coastal engineering. Two examples of these 

 problems are the protection of the cliffs at Newport, Rhode Island (U.S. 

 Army, Corps of Engineers, 1965) and at Gay Head, Martha's Vineyard, 

 Massachusetts. (U.S. Army, Corps of Engineers, 1970.) 



Most rocky shorelines are remarkably stable, with individual rock 

 masses identified in photos taken 50 years apart. (Shepard and Grant, 

 1947.) 



4.242 Beach Rock . A layer of friable to well-lithified rock often occurs 

 at or near the surface of beaches in tropical and subtropical climates. 

 This material consists of local beach sediment cemented with calcium car- 

 bonate, and it is commonly known as beach rock. Beach rock is important 

 to coastal engineers because it provides added protection to the coast, 

 greatly reducing the magnitude of beach changes (Tanner, 1960), and be- 

 cause beach rock may affect construction activities. (Gonzales, 1970.) 



According to Bricker (1971), beach rock is formed when saline waters 

 evaporate in beach sjinds, depositing calcium carbonate from solution. The 

 present active formation of beach rock is limited to tropical coasts, such 

 as the Florida Keys, but rock resembling beach rock is common at shallow 

 depths along the east coast of Florida, on some Louisiana beaches, and re- 

 lated deposits have been reported as far north as the Fraser River Delta 

 in Canada. Comprehensive discussion of the subject is given in Bricker 

 (1971) and Russell (1970). 



4.243 Organic Reefs . Organic reefs are wave-resistant structures reach- 

 ing to about mean sea level that have been formed by calcium carbonate 

 secreting organisms. The most common reef-building organisms are herma- 

 typic corals and coralline algae. Reef- forming corals are usually re- 

 stricted to areas having winter temperatures above about 18°C (Shepard, 

 1963, p. 351), but coralline algae have a wider range. On U.S. 



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